GCN4 is a transcriptional activator of amino acid biosynthetic genes in S. cerevisiae that is regulated at the translational level by upstream open reading frames (uORFs). uORFs 3 and 4 repress GCN4 translation under nonstarvation conditions; uORFI allows ribosomes to traverse uORFs 3 and 4 and translate GCN4 in amino acid-starved cells. We have strong evidence that ribosomes which translate GCN4 have previously translated uORF1, resumed scanning and bypassed uORFs 2-4. This occurs under starvation conditions because reinitiation of translation is inefficient in these circumstances. GCD factors are required for translational repression of GCN4. GCD1 and GCD2 appear to be involved in general translation and are physically associated with initiation factor -2 (eIF-2). In addition, mutations affecting eIF-2 subunits have a Gcd-phenotype. These findings suggest that antagonism of general initiation factors under starvation conditions leads to reduced reinitiation at uORFs 2-4 and consequent increased translation of GCN4. Antagonism of GCD factors requires the GCN1, GCN2 and GCN3 proteins. GCN3 is present in the GCD1-GCD2-eIF-2 complex and thus appears to act directly in modulating its function. This is consistent with the isolation of gcn3 mutations that derepress GCN4 expression in the absence of GCN1 and GCN2. GCN2 is a protein kinase that contains a domain homologous to histidyl-tRNA synthetases, which prompted the idea that its kinase function is activated by uncharged tRNA in starved cells. This suggestion is supported by the isolation of high copy-number plasmids containing tRNA genes that suppress a leaky gcn2 mutation. Increased GCN2 kinase function is thought to antagonize the GCD1-GCD2-GCN3-elF-2 complex and thereby stimulate translation of GCN4. A defective protein phosphatase gene (GLC7) was also isolated as a dominant suppressor of the leaky gcn2 mutation, suggesting that a balance between phosphorylation (by GCN2) and dephosphorylation (by GLC7) of initiation factors determines the efficiency of translational reinitiation. Ribosomal protein (rp) genes are repressed under amino acid starvation conditions. This repression is mediated by the binding site for the regulatory protein RAP1. RAP1 activation of rp genes is modulated by amino acid levels at a post-translational level, by a mechanism independent of GCN1-GCN4. Thus, separate pathways exist for regulating amino acid biosynthesis and ribosome biogenesis in response to amino acid availability.